1. Field of the Invention
The present invention relates to an over-coating composition and processes for forming a fine pattern using the same. In particular, the present invention relates to an over-coating composition comprising a basic compound, and methods for forming ultrafine (i.e., <150 nm) patterns using the same. The over-coating composition of the present invention is particularly suitable in a photolithography process using photoresist resins having a low light transmittance.
2. Description of the Background Art
In a photolithography process, an exposure of photoresist to light of a particular wavelength generates an acid from the photoacid generator present in the photoresist. The photo generated acid causes the main chain or the branched chain of the resin to decompose or become cross-linked. In addition, the acid removes the acid labile group and changes the polarity of the photoresist in the exposed region. This polarity change creates a solubility difference between the exposed portion and the unexposed portion in a developing solution, thereby allowing a pattern formation. The resolution of the pattern that is formed depends on the wavelength, i.e., in general, a shorter wavelength allows formation of more minute patterns.
Fine photoresist patterns of 150 nm L/S have successfully been developed using a light source which produces light having wavelength of 248 nm (KrF). Attempts at forming high quality fine circuit patterns smaller than 150 nm have thus far been relatively unsuccessful. These attempts have used light sources which generate short wavelengths such as ArF (193 nm), F2 (157 nm) and EUV (13 nm) and have employed photoresist resins having a low transmittance to the short wavelengths, resulting in poor quality patterns. For example, a photoresist resin which has been used with i-line (365 nm) and KrF (248 nm) light sources contains aromatic compounds, which have a relatively high absorbance of 193 nm wavelength light. Photoresists comprising acrylic or alicyclic resins which do not contain aromatic compounds have also been synthesized and used; unfortunately, these resins also have a relatively high absorbance of 193 nm wavelength light.
Use of conventional chemically amplified photoresist resins having a low transmittance is undesirable because the low transmittance results in more light reaching the upper portion of the photoresist than the bottom portion, which results in higher acid concentration in the upper portion of the photoresist than in the bottom portion of the photoresist, which can result in a bulk slope profile pattern. See FIG. 1b. This is contrasted to a pattern formed when the photoresist resin has a relatively low light absorbance. In this case, the amount of light reaching the upper and bottom portions of the photoresist is nearly identical, thus forming a desired vertical pattern. See FIG. 1a. 
In order to overcome the above disadvantages, efforts have been directed at synthesizing resins having a low light absorbance, in particular for light wavelengths of 157 nm (F2) and 13 nm (EUV). Unfortunately, these attempts have proven unsuccessful as acids generated in the exposed area of the photoresists are neutralized by atmospheric amines during the time between the exposure and post exposure baking (“post exposure delay effect”). Since pattern formation depends on acids that are generated by the exposure, neutralization of acids by atmospheric amine compounds reduces, prevents or alters a pattern or the pattern may have a T-shape (“T-topping”). This problem is especially acute when the concentration of environmental amines is over 30 ppb, which can lead to no pattern formation.